Recently, eco-friendly vehicles such as electric vehicles, hybrid and plug-in hybrid vehicles, and fuel cell vehicles have been mass-produced in order to improve the fuel efficiency in view of high oil prices and meet regulations on carbon dioxide emissions.
The hybrid vehicles among the above-mentioned eco-friendly vehicles can reduce exhaust gas and improve the fuel efficiency by selecting a motor and an engine as a power source. The hybrid vehicles require a high efficiency and performance power transmission system which transmits the engine power or the motor power separately or simultaneously to the drive wheels.
As shown in FIG. 1, a typical powertrain for a hybrid vehicle includes an engine 10 and a motor 12 that are directly connected to each other, and a clutch 13 that is disposed between the engine 10 and the motor 12 to transmit or interrupt the engine power. A controller 15 controls the clutch engagement and disengagement according to a driving situation. An automatic transmission 14 changes a speed of the power to output the power to a drive wheel, and an integrated starter generator (ISG) 16 is connected to a crank pulley of the engine to start the engine and generate electricity.
The engine clutch may be mounted between the engine and the motor instead of a torque converter to reduce the manufacturing cost of the hybrid vehicle and minimize a loss of the torque delivered to the transmission. Thus, the engine clutch may control the output torque of the engine.
Driving modes of the hybrid vehicles having the powertrain include an electric vehicle (EV) mode, i.e., a pure electric vehicle mode that uses the motor power only, and a hybrid electric vehicle (HEV) mode that uses the engine as a main power source and the motor as an auxiliary power source. The regenerative braking (RB) mode recovers vehicle braking or the inertial energy through the electricity generated in the motor during the vehicle braking or the inertia from driving the vehicle and charges the battery.
The HEV refers to a mode in which a vehicle is driven by the sum of the output torque of the engine and the motor when a main relay, which supplies power to the motor, turns on as the engine clutch is engaged during the driving situations, such as, start, acceleration, and climbing, that applies a great load to the engine.
In the EV mode, in which only the motor is driven, the engine clutch is disengaged, and the main relay turns on, thereby allowing the vehicle to run only by the output torque of the motor.
When the EV mode shifts to the HEV mode, the engine clutch is engaged. For example, the output speed of the engine and the speed of the motor are synchronized together with cranking of the ISG, and then, the engine clutch is engaged to shift the driving mode to the HEV mode.
In this case, when the EV mode (engine clutch disengaged) shifts to the HEV mode, a certain amount of time is consumed for the engagement of the engine clutch to shift to the HEV mode.
Accordingly, when the EV mode shifts to the HEV mode through the engagement of the engine clutch and the transmission to the lower gear, the acceleration performance at kickdown to accelerate the vehicle may largely depend on the response speed and the response performance of the engine clutch.
On the other hand, when the HEV mode shifts to the EV mode, the engine clutch is disengaged, and an entry condition for the disengagement of the engine clutch is as follows.
1) A case where the entry condition is satisfied by the ISG torque (ISG Tq)
2) A case where the entry condition is satisfied by an engine torque (engine Tq)
3) A case where the entry condition is satisfied by a vehicle speed and a gear shift
4) A case where the RPM of the motor satisfies the entry condition by the vehicle speed and a break depth
5) A case where the entry condition is satisfied by a shift phase of a transmission control unit (TCU) according to the vehicle speed, the gear shift, and the break depth.
Based on the foregoing entry conditions, FIG. 2 shows an engine clutch engagement condition.
In case of 1) or 3), when the ISG torque is equal to or smaller than table values by the current vehicle speed and the gear shift, the engine clutch disengagement condition is satisfied by the amount of torque.
In case of 1) or 3), when the engine output torque is equal to or smaller than table values by the current vehicle speed and the gear shift, the engine clutch disengagement condition is satisfied by the amount of torque.
When the foregoing two conditions are simultaneously satisfied, a time delay may occur according to the table values by the vehicle speed and the gear shift, and the engine clutch disengagement condition is satisfied.
In case of 4), when the RPM of the motor is smaller than the current vehicle speed and the brake depth, the engine clutch disengagement condition by the speed is satisfied.
In case of 5), when the vehicle speed is smaller than set values for each gear stage, and a brake switch is in an on-state, the engine clutch disengagement condition is determined by the shift phase of the TCU. Also, when the vehicle speed is greater than the set values for each gear stage or the brake switch is in an off-state, the engine clutch engagement condition is satisfied even when the shift phase of the TCU is not under shifting gear before stopping.
Thus, the clutch disengagement by torque is determined by input factors such as the engine torque, the ISG torque, the vehicle speed, and the gear shift, or the clutch disengagement by speed is determined by input factors such as an RPM of motor, the vehicle speed, and the brake depth. Also, the clutch disengagement is determined by the shift phase of the TCU.
When a driver decelerates a vehicle to change lanes, overtake other vehicles, or turn left or right at an intersection, the engine clutch disengagement condition is satisfied, allowing the engine clutch to be disengaged.
That is, during the HEV mode, when the driver confronts a situation where the vehicle needs to accelerate immediately after deceleration, such as changing lanes, overtaking other vehicles, turning left or right at an intersection, the driver usually decelerates the vehicle for safety. When the above-mentioned engine clutch disengagement entry condition is satisfied, the engine clutch is disengaged.
When the engine clutch is disengaged immediately after the deceleration in the HEV mode, the vehicle shifts to the EV mode. In this case, when a vehicle needs to accelerate, the engine clutch needs to be engaged, affecting the operability and accelerating performance of the vehicle.
As a result, when acceleration of a vehicle is needed immediately after deceleration, the engine clutch needs to be re-engaged for the EV kickdown operation for shifting from the EV mode to the HEV mode. In this case, since a period of time (about 1.1 seconds) is needed for re-engagement of the engine clutch, the operability and the accelerating performance of the vehicle may be affected, and the fuel efficiency may be reduced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.